Add simple user look up

The properties cmis:createdBy, cmis:lastModifiedBy, and cmis:versionSeriesCheckedOutBy as well as ACEs contain principal IDs. Some clients would like to know more about these principals. We should introduce a new (optional) operation getPrincipalInfo(), which accepts an array of principal IDs and returns a list of of principal infos. A principal info contains the principal ID, a principal type ("cmis:user", "cmis:group", "cmis:role", or repository specific type), a display name, a first name (optional), last name (optional), email address (optional), and, optionally, additional repository specific attributes.

(Do we also need an operation to find principals by other attributes, e.g. by email address?)